1. Field of the Invention
This invention relates to a process for cleaning the interior of a semiconductor substrate. More particularly, it relates to a cleaning process for removing rapidly-diffusible metallic impurities from the interior of a semiconductor substrate.
2. Description of the Prior Art
Contamination of semiconductor substrates with heavy metal impurities in the fabrication of MOS devices or bipolar structure devices on the surface of substrates of a semiconductor such as silicon often causes serious problems, such as the lowering of breakdown strength of the oxide film and the increase of leak currents through pn junctions. Cu and Ni, in particular, are so rapidly diffusible in silicon that they can cause great effects on the devices. Moreover, the contamination due to such metals is liable to occur during the operation of processing apparatus as well as before and after the operation.
Accordingly, as a technique for isolating such metallic impurities from device active regions of semiconductor substrates, methods called intrinsic gettering (IG) and extrinsic gettering (EG) are known in the art. These are methods in which crystal defect layers or thin-film layers capable of readily absorbing metallic impurities are provided in regions other than the device active regions so that the metallic impurities are made to move from the device active regions to these layers in the step of heating and are captured there.
Since the above gettering action is very strong in purification of the device active regions, any contaminant metallic elements coming from processing apparatus at each fabrication step are captured in the gettering sites more and more with the progress of processing. Especially in the steps of ion implantation and dry etching, the contamination may so much occur that the function of gettering may become saturated or closely saturated to result in a less effective gettering action with the progress of the process.
In addition, the metallic impurities once gettered may be liberated to again enter the device active regions. For example, elements, such as Cu that are fairly rapidly diffusible even at 300.degree. C. or below, are liberated from saturated or closely saturated gettering layers under some heating conditions during the processing.
In the IG, portions with a low oxygen concentration in the gettering layers have defects of a relatively low degree and hence combine with Cu so weakly that the metallic impurities are quite easily liberated (re-liberation) even at 300.degree. C. or below. Microdefects with a high oxygen concentration have a strong power to capture Cu, and hence such re-liberation is not significant below 300.degree. C. but easy at about 500.degree. C.
In the EG, when the layer that captures metallic impurities is a processing defect layer formed on the back, the re-liberation of metallic impurities tends to occur at a relatively low temperature. When the layer that captures metallic impurities is a polysilicon layer on the back, the re-liberation occurs in heating at a high temperature.
Hence, even when the gettering mechanism is provided, the impurities captured in the gettering regions are preferably as small as possible in amounts.